Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 02 2025 16:07:42 FOUS30 KWBC 021607 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1207 PM EDT Sat Aug 2 2025 Day 1 Valid 16Z Sat Aug 02 2025 - 12Z Sun Aug 03 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST, SOUTHERN PLAINS, AND NORTHERN PLAINS... ....Southern Appalachians through the Gulf Coast... A wavering hybrid cold/stationary front will remain draped from eastern Texas through South Carolina today, with a slow southern translation of this feature expected into Sunday morning. This front will waver (with the gradual southern progression) in a region of broad troughing extending down from the Northeast, with weak flow across the Gulf Coast leading to the slow movement. However, the front will be entrenched within a plume of elevated tropical PWs (measured via 12Z U/A soundings of 2.2 to 2.4 inches, nearing daily records across the area.) This will support widespread showers and thunderstorms along the low-level convergent boundary of the front, with generally weak west-to-east 0-6km mean winds of just 5-10 kts supporting slow storm motions, and training thanks to this boundary parallel wind. Weak impulses within the flow will enhance ascent leading to locally clustered convection with rainfall rates above 2"/hr (briefly 4"/hr likely as reflected by HRRR 15-min rainfall above 1"), and there are likely to be two primary areas of higher excessive rain risk. The first area is across the Southern Appalachians from northern Georgia through eastern TN and far southwest NC. Here, a narrow ribbon of instability 750-1000 J/kg (MUCAPE) will extend northward and interact with increasing moist upslope flow as 925-850mb winds veer more E/SE late this aftn into the evening. Although PW anomalies are somewhat lower here compared to points south, increased bulk shear and weak mean winds will support heavy rainfall rates of 2-3"/hr within what could be nearly stationary cells at times. 0-10cm soils in this region are saturated above the 95th percentile according to NASA SPoRT, leading to compromised FFG and a 60-80% chance of exceedance from the HREF. Farther south from eastern AL though GA and onto the SC coast, training of cells which develop along the front and within the greatest plume of PW will support an increased excessive rainfall risk, especially through peak heating. Although soils here are a bit drier due to a lack of recent rainfall (7 day rainfall generally 25-50% of normal), a 60-80% chance of 2"/hr rain rates from the HREF and REFS could lead to stripes of rainfall exceeding 5 inches (40-70% chance from both HREF and REFS.) After consideration of the new probabilities and 12Z CAMs, the two slight risk areas were merged into one larger SLGT risk which encompasses more of GA/eastern AL as signals from both the HREF and REFS were similar. ....Northern Rockies through the Southern Plains... An elongated north-south stationary front will drape from Montana through Texas today, while multiple mid-level impulses embedded within generally zonal flow from the Rockies into the Plains interact with this surface feature as they emerge from the west. At the surface, flow downstream (east) of this stationary front will become enhanced as return flow persists out of the Gulf, driving an 850mb LLJ to 30 kts across the Central Plains. This surging LLJ will help draw impressive thermodynamics northward, characterized by PWs of 1 to 1.25 inches (locally higher) overlapped with 1000-2000 J/kg MUCAPE. This will support widespread convection developing beneath any of these shortwaves, with development most likely along and just east of the wavering stationary front into the more impressive thermodynamics. Although convection is expected to move briskly eastward within any clusters that develop thanks to 30-40 kts of bulk shear, rainfall rates of 1-2"/hr are anticipated (30-50% chance from the HREF) which could cause at least isolated excessive rainfall impacts, especially across any urban areas or more sensitive soils with lower FFG. The greatest risk appears to be across the TX panhandle where an MCS may form tonight on the periphery of the greater instability plume and then dive into the moisture beneath one of these shortwaves, with a subtly higher training risk along its SW quadrant, and both HREF and REFS 3"/24 hr probabilities peak above 40%, and the inherited SLGT risk was adjusted cosmetically. There may be a secondary area of heavier rainfall across eastern SD and into ND where some repeating or backbuilding convection may occur later tonight as westerly flow in the vicinity of an MCV/shortwave results in repeating development into the higher instability west, and convection moves into some more sensitive soils in eastern SD tonight. This has prompted a targeted SLGT risk for tonight as well, which is supported somewhat by the CSU UFVS first guess field which has a SLGT risk in the same general area, although displaced from the HREF/REFS probabilities which drove this new excessive area. Weiss Day 2 Valid 12Z Sun Aug 03 2025 - 12Z Mon Aug 04 2025 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR SOUTHEAST GEORGIA AND FOR THE CENTRAL/SOUTHERN PLAINS... ....Southeast... The cold front continues on is journey southward, shifting the QPF footprint along with it. The heaviest rainfall still favors the far southern portions of South Carolina and southern Georgia. The Slight Risk was expanded westward to encompass more of southern Georgia. A broader Marginal Risk area covers the potential for isolated instances of excessive rainfall as far west as the Florida Panhandle and southern Alabama. Parts of northern Florida was trimmed from the southern bounds of the Marginal. ....Plains... MCSs will persist during this period with another model cycle showing a small eastward shift in the footprint of the heaviest QPF. Convection will be fueled by the deep influx of PW near 2 inches and enhance rainfall rates within these complexes. The higher potential for excessive rainfall and flash flooding concerns will likely be focused from south-central Kansas to southern Oklahoma. The Slight Risk area was maintained for this period as well as expanded on the eastern side across central Oklahoma. A broader Marginal Risk is in place from central Texas northward to eastern North Dakota and northern Minnesota. Campbell Day 3 Valid 12Z Mon Aug 04 2025 - 12Z Tue Aug 05 2025 ....THERE ARE MARGINAL RISK AREAS FOR THE NORTHERN ROCKIES, NORTHERN PLAINS/MINNESOTA AND THE SOUTHEAST... ....Southeast... Showers and thunderstorms will persist across the Deep South and Southeast during this period in proximity to the slowing cold front. Guidance suggests some of the heaviest QPF will be right along the Gulf Coast or over the Gulf. Areal averages up to 1 to 2 inches expected with locally higher accumulations possible. A Marginal Risk is in place and spans from southeast Mississippi to Florida Panhandle and northeast to South Carolina. ....Northern Plains... Organized convection will persist across the Plains during this period although with a much smaller areal extent than days previous. Parts of the region will be wet from recent rain over multiple days and will likely have lowered FFG. A Marginal Risk spans from the eastern Dakotas to western Minnesota and extreme northwest Iowa. ....Northern Rockies... Southwest flow aloft will persist during this period, priming the region for scattered thunderstorms. Abundant PW will lead to locally enhanced rain rates and accumulations across western Montana where recent rains have focused. A Marginal Risk area covers much of western and north-central Montana. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5Fww3peM_LtRgH1v4tl13vmLGK54agst5lGdHEOKsvpY= a_paCXFwhuYpvzM9_xydBN-cNSP3wTxuoWiVsn-uJ5wuYXM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5Fww3peM_LtRgH1v4tl13vmLGK54agst5lGdHEOKsvpY= a_paCXFwhuYpvzM9_xydBN-cNSP3wTxuoWiVsn-uWRo38Gs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5Fww3peM_LtRgH1v4tl13vmLGK54agst5lGdHEOKsvpY= a_paCXFwhuYpvzM9_xydBN-cNSP3wTxuoWiVsn-ueJVoRQo$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .